Hepatitis E occurs predominantly in developing countries of Asia and Africa but has also been the cause of epidemics in Mexico (1). The disease generally affects young adults and has a very high mortality rate, up to 20%, in pregnant women (1-4). Hepatitis E has rarely been reported in developed countries, and most of those cases have been imported (1, 4-6). The causative agent, hepatitis E virus (HEV), is transmitted primarily by the fecal-oral route, often through contaminated water (1, 4). The availability of sensitive serological tests for HEV has permitted detailed assessment of the prevalence of HEV infection (7-8). In regions where HEV is endemic, anti-HEV antibodies have been detected in sera from convalescent individuals as well as from the general population (1, 3). Although hepatitis E is not endemic in the United States and other developed countries, anti-HEV was found in a significant proportion, up to 28% in some areas, of healthy individuals in these countries (7, 9). It is unclear if the anti-HEV detected in developed countries demonstrates infection with a non-pathogenic HEV strain or cross-reactivity with a related agent.
It has been reported that anti-HEV is acquired naturally in primates and swine (1, 10), suggesting that these species have been exposed to HEV or a related agent, and that hepatitis E may be a zoonotic disease. The role of swine in HEV transmission is not clear although domestic swine have been reported to be susceptible to infection with a human HEV strain (11). It would be advantageous to have an animal strain of HEV that could be studied in animals but would sufficiently resemble the human virus as to make the results of studies with the animal virus applicable to the human disease. It would be particularly advantageous to have an animal virus that could be useful in preparing vaccines and other medicaments for treatment of animals, especially humans.
It has been reported that domestic swine can be experimentally infected with HEV (11), although this infection appears more severe in swine than in non-human primates. In our own examination, swine inoculated with human HEV remained clinically normal and we detected no antibody response to HEV. While it has been suggested that human feces from infected individuals are the primary source of infection for hepatitis E in humans, swine have been suggested as a possible animal reservoir and natural host for the virus. For example, in some geographical areas the prevalence of anti-HEV in swine is even higher than that found in humans and swine have been suggested as a possible year round reservoir for HEV (10). However, the occurrence of IgG anti-HEV in swine does not necessarily mean that HEV infection has occurred. Hepatitis E is not endemic in the United States and yet our own studies have indicated that the majority of swine 2 to 3 months of age in the midwestern United States have IgG anti-HEV. In addition, IgG anti-HEV is also found in a significant proportion of healthy humans in countries, including the United States, where hepatitis E is not endemic (7). Such data indicated to us that an agent or agents other than human HEV, but antigenically closely related to human HEV, exists within the swine population. The antibody induced by this putative agent has been shown to cross-react with a human HEV antigen used in serological testing. Such findings mean that caution must be used in interpreting serological data, especially from non-endemic regions. Identification of such a putative agent would, of course, be highly advantageous in order to develop more specific serological tests for HEV infection. The detection and isolation of such a virus from swine would also be desirable in determining the possible existence of an animal reservoir for HEV. In addition, such a virus, if capable of infecting swine but not causing illness, might be well suited for use as a vaccine or therapeutic agent for use in vaccinating and treating mammals, especially humans. Thus, such a virus could well serve as an attenuated live virus vaccine strain.
The present invention is directed to a novel swine hepatitis E virus (swine HEV) strain in isolated form and with sufficient nucleotide sequence homology in its genome, and sufficient amino acid sequence homology in its capsid protein, both relative to human HEV, as to be highly useful in the evaluation of potential human infection by endogenous swine sources, including potential infection resulting from xenotransplantation, especially transplanting of swine organs and tissues into humans.
The swine hepatitis E virus of the present invention is also useful as a vaccine for vaccination of animals, preferably mammals, and most preferably humans, against infection by other strains of hepatitis E virus. The swine HEV of the present invention would, for example, be especially useful as a vaccine for use in humans to prevent possible infection by human hepatitis E virus (or HEV). The swine HEV of the present invention accomplishes this by providing only a subclinical infection on its own and thereby immunizing the subject animal, for example, a human, in, for example, a manner similar to that accomplished by cowpox virus in immunizing against smallpox.
The present invention also relates to use of a novel swine HEV as a therapeutic treatment for infection by other strains of HEV by injection of the virus of the invention to bolster the immune response of an infected animal while providing only a subclinical infection on its own.
The swine HEV of the present invention is also highly useful in the generation of bosh polyclonal and monoclonal antibodies which themselves find use as therapeutic agents for treatment of animals, especially mammals, and most especially humans, in need thereof.
The antibodies produced in response to the swine HEV of the present invention also find use in the development of in vitro diagnostic protocols for early detection of HEV infection in animals, especially mammals, and most especially humans.
The swine HEV of the present invention is particularly advantageous for use in the development of prophylactic, therapeutic and diagnostic agents for the prevention, treatment and detection of human HEV because it is not a human virus and thus can be handled both experimentally and clinically without fear of severe infection and/or contamination.
SPF swine were experimentally infected with the swine HEV of the present invention. Therefore, infection of swine with the swine HEV can provide an appropriate animal model for human HEV experiments.